Zeolites are highly crystalline materials containing tetra-coordinated aluminum atoms, each associated through four oxygen atoms with adjacent silicon atoms in the crystalline matrix. These zeolites tend to have large particle sizes. For example, commercially available zeolite 4A has a particle size of about 3 to about 6 microns. Furthermore, due to the microporous structure of the zeolite, the material absorbs moisture. Therefore, often times the zeolites are used as adsorbents. Additionally, zeolites are used as catalysts. Various processes have been used to form zeolites.
For example, U.S. Pat. No. 3,528,615 describes a method of reducing the particle size of crystalline zeolites. In this process, the zeolite is heated to an elevated temperature. This temperature is below the temperature at which the loss of crystallinity occurs. The heated zeolite is then quenched in a liquid medium which is maintained below the elevated temperature. The thermal shock fractures the zeolite to produce smaller crystals without a significant reduction in the crystallinity.
Another example of a prior art process for zeolites is disclosed in U.S. Pat. No. 4,581,214. This patent discloses a shock calcination method. According to this method, the zeolite is precalcined at a relatively low temperature. The zeolite is then heated to a relatively high calcination temperature for a relatively short period of time because prolonged exposure of the zeolite to the high temperature would destroy the original structure of the zeolite. The patentee notes that it is very important to prevent mineralization reactions from occurring by this rapid heating steps. The zeolite is then rapidly cooled. The process described in U.S. Pat. No. 4,581,214 alters the zeolite surface acidity. This zeolite is then used as a catalyst.
Zeolites are effective acid scavengers for halogen containing polymers and enhance the thermal stability of halogen containing polymers. Acid scavengers are compounds that react with acids to form a compound that is typically chemically inert. However, the use of zeolites as stabilizers or acid scavengers in halogen containing polymer compounds has been limited for several reasons. First, the zeolites generally have a large particle size, generally in the range of about 3 to about 6 microns. The large size of the zeolite particles not only causes surface blemishes on the finishing of the end product made from such a polymer but also diminishes the physical properties of such polymers. Further, outgassing occurs frequently with polymers containing zeolites when the polymer is heated during processing due to the evolution of water from the zeolite during the heating. As a result, there is foaming.
Halogen containing polymers tend to degrade or deteriorate when processed. Generally, the difference between the processing temperature and the degradation temperature is very small. Therefore, there is a risk that during the processing of these halogen containing polymers, that the polymer will degrade. When such polymers degrade, it is believed that a halide acid is generated by the polymer. This acid attacks the components of the processing equipment. Also, this acid further catalyzes elimination reactions and additional degradation of the polymer.
Stabilizers have been developed to help deter such degradation. For example, organic compounds are commonly used. In some instances, zeolites have also been used as stabilizers.
U.S. Pat. No. 4,000,100 discloses a thermal and light stabilized polyvinyl chloride resin. The stabilizer used in the composition comprises an unactivated zeolite A molecular sieve or an unactivated naturally occurring molecular sieve of essentially the same pore size range as zeolite A and a conventional inorganic, organometallic or organic stabilizer. The unactivated zeolite molecular sieve has adsorbed water molecules. According to the patentee, the combination of the unactivated zeolite and the conventional stabilizer produces a compound with allegedly improved stability as compared to a compounds produced with either of the two stabilizers separately.
Similarly, U.S. Pat. No. 4,338,226 discloses a process for the stabilization of polyvinyl chloride and stabilizer compositions. The patent describes admixing sodium aluminosilicate of small particle size (preferably, 0.1 to 20 microns), calcium salts of fatty acids, zinc salts of fatty acids, partial esters of polyols and fatty acids, thioglycolic acid esters of polyols and polyvinyl chloride or copolymer of vinyl chloride. An aluminosilicate that can be used is crystalline sodium zeolite A. The composition is used for molding mixtures.
U.S. Pat. No. 4,371,656 describes a metal substituted zeolite for use as a stabilizer for halogen containing resins. The stabilizer comprises a crystalline aluminosilicate substituted with ions of metallic elements belonging to Group II or Group IVA of the Periodic Table for the Group I (M) metal ion contained in the aluminosilicate. The stabilizer also must contain 10% by weight or less as M.sub.2 O of residual Group I metal ions. The stabilizer, zeolite A, according to the patentee claims to have a water content of 8% by weight or less. This patent also discloses the use of organic substances to cover the voids of the zeolite particles and prevent moisture reabsorption.
Stabilized chloride containing resins are also described in U.S. Pat. No. 5,004,776. The stabilizer consists essentially of: (a) an overbased alkaline earth metal carboxylate or phenolate complex; (b) zeolite; (c) calcium hydroxide; and (d) a complex of at least one metal perchlorate selected from the group consisting of sodium, magnesium, calcium, and barium perchlorates with at least one compound selected from the group consisting of polyhydric alcohols and their derivatives. This stabilizer apparently prevents the discoloration and deterioration in physical properties of the chlorine containing resin resulting from thermal degradation when the resin is subject to thermoforming or exposed to a high temperature atmosphere for a long period of time.
Stabilizer compositions for use in halogen containing polymer are also described in U.S. Pat. No. 5,216,058. The stabilizer composition comprises hydrotalcite and a molecular sieve zeolite. The molecular sieve zeolite comprises a Group IA or IIA aluminosilicate.
Thus, there currently exists a need for an zeolite having a small particle size and a narrow particle size distribution. Furthermore, a need exists for a modified zeolite having a reduced water content while retaining reactivity with acid species. Preferably, the water content is less than 10 weight percent. The modified zeolite can be comprised a zeolite having a mean diameter greater than 1.5 microns with a narrow particle size distribution, or a zeolite having a mean particle diameter less than 1.5 microns and a narrow particle distribution, or a mixture of the two. In addition, a need exists for a modified zeolite which maintains its stabilizing activity. Moreover, a need exists for a method to form such small particle size zeolite and modified zeolite.